The invention relates to a tensioning device for traction means, such as a belt tensioning device, having a receiving and mounting housing, a roller carrier pivotably connected to the receiving and mounting housing, a bearing which radially and axially supports the roller carrier relative to the receiving and mounting housing, a spring for applying the tensioning force, and a damping device. The spring is arranged between the roller carrier and the receiving and mounting housing and the damping device comprises a damping bush. By friction, the damping device dampens any movements between the roller carrier and the receiving and mounting housing in such a way that the damping effect is greater in the direction of an increasing tensioning force than in the direction of a decreasing tensioning force.
In motor vehicles, the vehicle engine drives various auxiliary units such as a generator, a water pump for vehicle engines with fluid cooling, a pump for servo-power for motor vehicles with steering assistance and a compressor for an air-conditioning system. Such auxiliary units are driven by a main belt pulley positioned on the crankshaft of the vehicle engine and by an infinite driving belt which, on the one hand, moves via the main belt pulley and, on the other hand, moves via auxiliary belt pulleys arranged on the driveshafts of the auxiliary units. The functional efficiency of the auxiliary units and the service life of the driving belt greatly depend on the setting and maintaining of a certain belt tension of the driving belt. To improve the functional efficiency of the auxiliary units and to prolong the service life of the driving belt, it is general practice to use a spring-loaded belt tensioning device which, as a rule, comprises a damping device, so that any slip and vibrations can be compensated for or reduced.
It is mentioned above that the invention relates to a tensioning device for traction means. Above, details are given of a special tensioning device for traction means, i.e. a belt tensioning device. Below, the tensioning device for traction means, which is dealt with by the invention, will always be referred to and described as a belt tensioning device. This does not alter the fact that the described tensioning device for traction means can also be used for other applications, i.e. not only as a belt tensioning device.
The initially described basic design of belt tensioning devices, i.e. comprising a receiving and mounting housing, having a roller carrier which is pivotably connected to the receiving and mounting housing, bearings which radially and axially support the roller carrier relative to the receiving and mounting housing, a spring which applies the tensioning force and is arranged between the roller carrier and the receiving and tensioning housing, as well as a damping device, is generally known and to that extent, reference is made to the German publication documents 37 28 158; 41 24 636; 41 34 354; 43 36 467; 43 45 150; 44 27 683; 195 40 706 and 196 03 558, to the German patent specifications 43 00 178 and 43 25 424, to the European publication documents 0 294 919 and 0 858 563 as well as to the U.S. Pat. Nos. 4,698,049 and 4,472,162. Substantial differences, in particular, refer to the damping device.
In the case of the belt tensioning devices known from the German publication documents 195 40 706 and 196 03 558 and from the European publication document 0 858 563, the function of the bearing and that of the damping device are combined in one assembly. The assembly is a cone-type friction bearing which is applied by a spring force and which, on the one hand, radially and axially supports the roller carrier relative to the receiving and mounting housing and, on the other hand, by way of friction, dampens the movements between the roller carrier and the receiving and mounting housing. The disadvantage of the prior art belt tensioning devices is that, on the one hand, due to the unavoidable wear of the cone-type friction bearing, the roller carrier moves axially relative to the receiving and mounting housing and, on the other hand, the damping effect in the direction of load application, i.e. in the direction of an increasing tensioning force, is identical to the damping effect in the direction of load relief, i.e. in the direction of a decreasing tensioning force.
In the belt tensioning device known from the European publication document 0 294 919, it has been possible to avoid the two above-mentioned disadvantages which are inherent in the belt tensioning devices known from the German publication documents 195 40 706 and 196 03 558 and from the European publication document 0 294 919. As the belt tensioning device known from the European publication document 0 294 919 does not comprise a cone-type friction bearing, the disadvantage relating to such a cone-type friction bearing, i.e. that the roller carrier moves axially relative to the receiving and mounting housing, has been avoided. In the belt tensioning device known from the European publication document 0 294 919, the damping device is designed in such a way that the damping effect in the direction of the increasing tensioning force is greater than in the direction of the decreasing tensioning force.
As far as details are concerned, the damping device used in the belt tensioning device according to the European publication document 0 294 919 comprises a damping bush and a band which is positioned around the damping bush, whose first end is rigidly connected to the receiving and mounting housing and whose other end is subjected to a spring force. The spring force for tensioning the band is applied either by an additional helical spring on the receiving and mounting housing or by the spring primarily serving for applying the tensioning force. In the embodiment shown, it is a coil spring in the form of a leg spring. The first variant is disadvantageous in that there is a need for considerable additional means, in particular, the additionally required helical spring. The second variant is disadvantageous in that the design of the spring which serves to apply the tensioning force also determines the damping characteristics of the belt tensioning device. The degree of freedom desirable for the damping characteristics is thus missing.